(1) Field of the Invention
The present invention relates to semiconductor read only memory (ROM) devices, and more particularly to a mask ROM device having not and (NAND) type high integration double polycrystalline silicon word lines and the process for producing.
(2) Description of the Invention
A mask ROM is a read only memory device that is unable to change data after the data is filed by a process of making code implants, i. e. implanting impurity ions to alter the threshold voltage of selected memory cells. The ROM device structure is simple, and the degree of integration is the highest among semiconductor memory devices. These factors have make the mask ROM a very popular device for various computer and other related applications.
Mask programed ROM's are known in the art and used in various applications. U.S. Pat. No. 5,081,052 discloses a ROM which consists of an array of FET's arranged in columns and rows, interconnected by word and bit lines. At each intersection of the word and bit lines, is a FET, which constitutes a single memory cell. These FETs can be tailored to be either conducting or non-conducting, which can arbitrarily be assigned a 1 or 0 value. The tailoring of the memory cells can be done by programming, where the FET includes a "floating gate", or by selectively ion implanting impurity ions into the channel area of the FET, thereby changing the device channel region from a depletion mode to an enhancement mode. U.S. Pat. No. 5,081,052 describes this type of mask ROM device and the procedure for "code implanting" to tailor the device array. U.S. Pat. No. 5,094,971 describes a generally similar type ROM device.
An important objective in fabricating memory devices is to increase the miniaturization in order to increase the number of memory cells in the device. As the size of the individual cells decreases, the implantation of impurities (code implants) becomes more difficult. The definition of the necessary openings in a resist layer mask, becomes harder to achieve, as well as the registration of the mask used to expose the code pattern in the resist. The code implant must be located very accurately in order that final implant after diffusion does not overlap another memory cell. What is needed in order to increase miniaturization of ROM memories, are better techniques to code implant, and improved techniques to fabricate smaller word lines.